I can see where this is a problem for the estate. You could get hurt or damage something and the permission of the previous owner is probably either void or not supported by documentation. You hunting on their land could possibly open them up to a lawsuit. Maybe once it is sold, your agreement could be pursued again with the new owner.
You could try contacting family members and mention the water/hunting agreement, and maybe they would put it in writing...but I'd doubt that after hearing some stories that mimic yours. In fact, I was the "victim" of land passing through children. We had quite a bit of money invested in a lease and there was an agreement that we would have the property indefinitely. Two water wells, roads built with gravel, drainage and thousands worth of dirt work, etc....all paid for by us. The owner was a long time family friend, fellow church member, and neighbor. The grandfather/owner of the property passed away. The sons all agreed to continue the lease to us and things went uninterrupted for the first season. In fact, we built two more ponds and put in two new 60" corrugated metal culverts. The following summer, the lease manager was contacted by a law firm asking us to cease activity on the lease until a court proceeding was finalized. Turns out, a long-lost aunt of the boys and sister of the owner that passed away had some stake in the land from their father, and her daughter in Chicago owned half of the property. She immediately sold her half (the best half for hunting) for a cut-rate price to a large land management firm, who then posted it for 4x what they paid. We were allowed to continue hunting on the other half, but that was poor hunting land and didn't even continue the lease. One of the members was so pissed that he brought his dozer out and flattened several levees, poured concrete down the wells, and crushed the culverts. It was a great property to hunt and I miss it regularly. I wasn't even able to get any stands or feeders out either, because soon after it flooded and the crushed culverts washed out due to low flow and you couldn't get to the back without a tank.